if a grid cell (13x13, 26x26, 32x32) is already reserved for an object, new object cannot be assigned on it.
this is one of reason why YOLO is doing 3 scale predictions (even if objectA is missed on prediction one, prediction two or three can cover it)
each grid cell predicts only one object. https://github.com/jl749/YOLOv3/blob/4f9c1764991d0dbe23ad60e821bc376476c8fd2f/yolov3/datasets/pascal_VOC.py#L111-L113 these 3 lines of code will throw assertion error.
if a grid cell (13x13, 26x26, 32x32) is already reserved for an object, new object cannot be assigned on it. this is one of reason why YOLO is doing 3 scale predictions (even if objectA is missed on prediction one, prediction two or three can cover it)